Morphologies of Si phase and La-rich phase in as-cast hypereutectic Al–Si– xLa alloys

Abstract

In the present study, the diversified morphologies of Si phase and La-rich phase in as-casted hypereutectic Al–Si– xLa alloys are presented and investigated. The morphological features were examined using conventional optical microscopy and SEM for observations conducted on the optical samples and deep-etched samples, respectively. The results show that primary Si crystals show several morphologies, such as feathery, star-shaped, faceted polygonal, platelet and so on. There are three types of fivefolded Si crystals existing in the present study, fivefold symmetry as radial growth alone: thin-branched, coarse-branched and well-defined star-shaped growing from the preferred growth from the tips of branches. The eutectic Si in unmodified Al–Si alloys appears only in fibrous morphology, while discrete and interconnected coral and rodlike eutectic Si particles were observed in alloys with the addition of La. The La-rich phase also grows into a variety of morphologies, such as needlelike, broken rodlike in pores, spherical, and flat platelet. In optical microscopy, La-rich phase is observed to envelope some small polygonal Si crystals.